The C-13 chemical shifts delta of carbonyl (C-13=O) and nitrile ((13)Cequivalent toN) compounds in aqueous solutions were measured as functions of temperature to study the hydration structure around dipolar solutes. The delta values showed an anomalous maximum in the temperature dependence; at -9 degreesC for urea, at 23 degreesC for diethyl ketone, at 24 degreesC for cyanamide, at 32 degreesC for acetone, at 63 degreesC for acetaldehyde, and at 96 degreesC for acetonitrile. No such anomaly was observed in any other organic solvents. The presence of the maximum is due to the competition of two factors, solute-solvent and solvent-solvent interactions. One is the electrostatic hydrogen bond between the solute and water, and the other is the hydrogen bond network characteristic of solvent water. When the network structure of water is collectively stabilized with decreasing temperature, the effective water-water interaction becomes stronger than the solute-water interaction, giving the maximum in the temperature dependence of delta. The temperature of the maximum chemical shift T-max delta was found to be sensitive to the solute polarity. The solute dependence of T-max delta was explained in terms of the solute dipole moment mu obtained from ab initio calculations: the larger the mu, the lower the T-max delta.